The present invention relates to earth-supported hollow piles (“caissons”), for example, as are used as foundations for towers and the likes and in particular to a caisson providing reduced installation time and cost.
Construction projects, for example, those routing high-voltage electrical transmission lines, may require placement and setting of a large number of towers to support high voltage electrical conductors safely above the ground free from interference. The foundations for these towers may be provided by tubular steel caissons embedded in the ground to be supported by the surrounding earth. The tubular form of these caissons provides for great strength and the open lower ends offer low resistance to the caisson being driven downward through the earth which may pass along the inside and outside of the tubular steel walls. Accordingly, when soil conditions are right, caissons are normally installed by driving them directly into the earth without first preparing a hole.
Driving caissons directly into the ground may be done with a vibratory hammer applying a rapid series of high force impacts to the top of the caisson typically through a specially installed protective cap fitting over the caisson end. The vibratory hammers have internal eccentric weights, for example, driven by a hydraulic motor and have a hydraulic clamp that may clamp the vibratory hammer tightly to the protective cap and caisson to directly couple forces from the vibratory hammer into the caisson walls. The vibratory hammers are normally associated with a large weight providing an inertial backstop against which the hammer may operate. This weight is coupled to the vibratory hammer with an asymmetric elastomeric coupling that promotes high downward forces yet attenuated upward forces so that the net progress of the caisson moves downward during vibration.
The current process for installing a caisson using a vibratory hammer may require a crew to install the protective cap on the caisson and an on-site crane to lift the caisson into vertical orientation. A second crane holding the vibratory hammer may then be positioned above the caisson and clamped to the protective cap to drive the caisson into the earth. The protective cap is then removed and the tower installed on the portion of the caisson projecting above the ground. This process is repeated for each caisson to be installed with a typical project requiring many hundreds of caissons.